You can set the background color and transparency for the label in the following way:
label.backgroundColor = UIColor(red: 0, green: 0, blue: 0, alpha: 0.7)
To make your background darker set the alpha (transparency) to a higher value. Completely opaque is an alpha of 1.0 and completely transparent is an alpha of 0.0. So experiment around a bit with the alpha until you find a value you like.
Of course you can also adjust the red, green and blue values to a value of your liking. An RGB of 0,0,0 will make the darkest option, black, which may be the most useful for you here.
Edit: You can also use the following alternatives to achieve the same effect:
label.backgroundColor = UIColor(white: 0.0, alpha: 0.7)
label.backgroundColor = UIColor.blackColor().colorWithAlphaComponent(0.7)
Edit: In Swift 3 this would be:
label.backgroundColor = UIColor.black.withAlphaComponent(0.7)